Advanced composites are high strength, high modulus materials which are finding increasing use as structural components in aircraft, automotive and sporting goods applications. Typically, they comprise structural fibers such as carbon fibers in the form of woven cloth or continuous filaments embedded in a thermosetting resin matrix.
Most advanced composites are fabricated from prepreg, a ready-to-mold sheet of reinforcement impregnated with uncured or partially cured resin. For most uses, prepreg must possess a special balance of physical and chemical properties, particularly including the ability to lightly adhere to a metal surface or other plies of prepreg (tack) and the ability to be folded around corners having a radius as small as 1/4 in without damage (drape). Tack and drape are particularly important for use in laying up large complex parts such as aircraft wings or fuselages. The reactivity and, hence, the shelf or storage life of prepreg is also important. For example, most aircraft applications require prepreg with low reactivity at room temperature that will retain acceptable tack and drape for at least 10 days, yet be rapidly curable at elevated temperatures (i.e., in 2 to 4 hr at 350.degree.-470.degree. F.).
Resin systems containing an epoxide resin and aromatic amine hardener are often used in prepreg since they possess an appropriate balance of properties. State-of-the-art composites made with these formulations have high compressive strengths, good fatigue characteristics and low shrinkage during cure. However, most epoxy formulations absorb as much as 5 to 6 wt% moisture which causes property deterioration at elevated temperatures and reduces the dimensional stability of cured composites. As a result, they are not suitable for use at 130.degree. C. or greater in a moisture-saturated condition. Since most epoxy formulations used in prepreg are also brittle, the resulting composites have low toughness and poor impact resistance.
Other curable resin systems are known for these applications. For example, in U.S. Pat. No. 3,562,214, resins comprising aromatic cyanic acid esters and polyepoxide compounds are disclosed. The cyanic acid ester and the epoxide compound are generally heated together to provide a homogeneous liquid which can be cast into molds. Also disclosed is the use of solutions of these components in a solvent such as acetone, methyl ethyl ketone and the like, for impregnating webs of paper or fabric which can be employed in making laminates.
Also useful are the compositions described in U.S. Pat. No. 4,110,364 that comprise a cyanate ester, a bismaleimide, and, optionally, an epoxy compound; and the compositions described in U.S. Pat. No. 4,157,360, which include a cross-linked cyanate polymer and a thermoplastic polymer. The latter compositions may be prepared from a solution of the thermoplastic polymer and aromatic dicyanate monomer by removing the solvent, then curing the resulting intimate mixture by heating at a temperature above about 200.degree. C. The compositions may also be prepared from a melt containing the polymer and monomer by heating at 200.degree. C. or higher to yield a partially cured or cured composition. These compositions, typically containing equal parts by weight of the thermoplastic polymer and the cyanate monomer, do not possess the appropriate tack and drape required generally for many prepreg applications.
A curable composition comprising a polyphenylene ether resin, a bismaleimide and/or a cyanate ester and an epoxy compound is disclosed in U.S. Pat. No. 4,496,695, together with copper-clad laminates comprising such formulations reinforced with glass fabric. However, since polyphenylene ethers are high temperature, high melt-viscosity resins and are insoluble in these liquid components, the compositions can only be prepared by first dissolving the components in a suitable solvent.
Although formulations based on bismaleimides and epoxy resins are thus known, improved formulations useful in producing fiber-reinforced composites having good toughness and the ability to retain stiffness properties after exposure to hot/wet conditions are needed.